1. Field of the Invention
The present invention relates to an injection molding die apparatus for use in the production of a substrate of an optical disk. More particularly, the present invention is concerned with an injection molding die which enables a uniform separation of an optical disk substrate from a stamper on the die after injection molding of the substrate.
2. Description of the Related Art
In general, an optical disk has a substrate which is formed from a plastic material by injection molding. Injection molding of the substrate of an information recording medium of this kind usually employs a stamper which is provided in the injection molding die so that information recording tracks are transferred to the substrate from the stamper simultaneously with the formation of the substrate. After the formation of the substrate and the transfer of the track grooves, the substrate is separated from the stamper. The separation has to be conducted with greatest care in order to avoid any damage of the track grooves so as to ensure that the track grooves can function satisfactorily, and various proposals have been made to enable safe separation of the substrate.
FIG. 4 shows an example of a known injection molding die for use in the production of an optical disk substrate. The injection molding die has a stamper 1 fixed to a movable die part 2 which is adapted to be moved towards a stationary die part (not shown). A molten plastic material is injected into a space defined by the stationary die part, stamper 1 and an outer peripheral ring 3 provided on the outer periphery of the stamper 1, whereby a substrate 4 of optical disk is formed. A through-hole 5 is formed in the center of the movable die part 2 such that the axis of the through hole 5 extends in the direction of movement of the movable die part 2. The through-hole 5 loosely receives a sleeve-like ejector member 6 connected to an actuator (not shown). After the injection molding of the substrate 4 of the optical disk, the movable die part 2 is moved away from the stationary die part for the purpose of parting. In consequence, the optical disk substrate 4 is separated from the stationary die while being held in close contact with the stamper 1. At the same time, the ejector member 6 is projected from the stamper 1 so as to push the central portion of the optical disk substrate 4 held in close contact with the stamper 1, thereby separating the optical disk substrate 4 from the stamper 1. In addition, air is introduced into the space between the optical disk substrate 4 and the stamper 1 through an annular air slit 7 formed between the ejector member 6 and the through-hole 5 which loosely receives the ejector member 6, thereby completely separating the optical disk substrate 4 from the stamper 1. Although the separation in the illustrated case is conducted by cooperation of the mechanical action of the ejector member 6 and the pneumatic force produced by the air, the ejector member 6 is omitted in some known apparatus so that the separation is conducted solely by the flow of air introduced through the air slit 7. Referring to FIG. 4, a rod 6a is inserted into the sleeve-like ejector member 6. Namely, only the portion of the ejector member 6 contactable with the optical disk substrate 4 has a cylindrical form.
Thus, in the known injection molding die for use in the production of an optical disk substrate, the optical disk substrate 4 formed by injection molding, still held in close contact with the stamper 1 held by the movable die part 2, is first separated from the stationary die part and then separated from the stamper by the cooperation between the mechanical action of the ejector member 6 and the pneumatic force produced by the air introduced through the slit or by such a pneumatic force a alone. Thus, the separation of the optical disk substrate takes place first at the center of the optical disk substrate 4 and progressively propagates to peripheral regions. This known separation technique tends to cause problems such that the track grooves left after the separation are irregularly deformed or the optical disk substrate 4 itself is distorted, with the result that the performance of the optical disk as a product is undesirably impaired.
More specifically, when the optical disk substrate 4 which is still hot is separated from the stationary die part, only the surface which has been held in contact with the stationary die part is rapidly cooled due to contact with air and is contracted with the result that the optical disk substrate 4 is deflected such that the central region of the optical disk substrate 4 is convexed towards the stamper 1 with the peripheral portions of the optical disk substrate 4 quickly separated from the stamper 1. Subsequently, parting force is applied by the ejector member 6 and the compressed air supplied through the air slit 7 to the central portion of the optical disk substrate 4 so that the central portion of the optical disk substrate 4 is recessed away from the stamper 1 with radially intermediate portion of the substrate 4 convexed towards the stamper 1. In consequence, slight radial displacement of the optical disk substrate 4 is caused relative to the stamper 1 with the result that the transferred track grooves are deformed in a manner shown in FIGS. 5 and 6. Referring to these figures, numeral 8 denotes minute pits transferred from the stamper 1 to the track surface 4a of the optical disk substrate 4. The pits 8 are formed at a predetermined pitch in the region of the track surface 4a between adjacent circumferential grooves 9 which are formed by the transfer. The above-mentioned radial displacement of the optical disk substrate 4 with respect to the stamper 1 at the time of separation causes that radial drags of the pits are formed as hatched in FIG. 5, thus leaving transfer failure portions 8a. Such transfer failure portions 8a may take place over the entire area of the track surface 4a. An optical disk having a substrate 4 involving such transfer failure exhibits inferior optical characteristics due to double refraction of a laser beam applied thereto. In addition, the deformation of the optical disk substrate 4 itself causes a predetermined allowable inclination of disk surface to be exceeded, result in degradation of performance of the optical disk.